Signals transmitted at high frequency over conductive traces are subject to transmission characteristics of the transmission medium they traverse. For example, the transmission characteristics includes transmission line effects, frequency responses of connectors, etc. For example, printed circuit board traces do not transmit signals instantaneously from transmitting device to receiving device, but have an associated propagation time, for example, one to two nanoseconds of propagation time for a trace of 100 to 200 mm in length. In these circumstances, the transmitted signal when received at a receiving device can be distorted due to various transmission line effects. At high data transmission rates, such distortion can prevent transmitted signals from being properly interpreted by receiving devices. Such transmission line distortion can be manifested by frequency dependent roll-off, reflections, ringing, cross talk, and other types of signal distortion. FIG. 1 illustrates signal distortion as can occur with known output drivers.
In FIG. 1, an output driver 12 receives a data signal 10 having transitions between a high state and a low state. The output driver 12 generates output signal 14, which is a representation of the original data signal 10. Output signal 14 is transmitted over transmission medium 16 to a receiving device (not shown). However, due to transmission line effect distortion the output signal 14 is distorted, as represented by received signal 18, when it is received at the receiving device.
A number of methods have been developed to compensate for distortion caused during the transmission of a signal or minimize the distortion. One method used to reduce transmission line distortion is to improve the characteristics of the transmission medium that affect the transmission of signals. For example, FR-4 laminate material commonly used with printed circuit boards tends to negatively affect high frequency characteristics of transmitted signals, especially as the length of the conductive traces increase. For example, above a transmission frequency of 1 gigahertz the dielectric characteristics of normal FR-4 laminate in combination with the conductive trace result in significant signal distortion associated with transmission line effects. Accordingly, one method of improving the high-frequency characteristics of a system is to use printed circuit boards made of materials other than FR-4 that better isolate conductive traces, and therefore reduce distortion due to transmission line effects. However, such alternate PCBs are very expensive in comparison to FR4.
Another common method of compensating for signal distortion is to use a series connected pre-emphasis circuit within the transmitter and/or a series connected equalizer in the receiver. However, the use of pre-emphasis circuits and/or equalizers often increases the complexity of the system, making it more costly and/more difficult to implement. In addition, the pre-emphasis circuitry and equalizer increases the consumption of power.
Signals may still be detected in the presence of certain type of transmission related distortion using various methods of ‘partial response detection’. These detectors attempt to resolve the most likely transmitted data based upon recently received data and characteristics of the transmission medium. A ‘Viterbi’ detector is a circuit of this type. These complex circuits are often quite costly, however, and can almost invariably benefit in performance from having reduced transmission related distortion. Hence, it is often the case that even systems employing a partial response circuit may employ other schemes to optimize the overall channel performance. It is apparent that an improved device and method for compensating and/or reducing the effects of signal distortion that overcome limitations of the known art would be advantageous.